In recent years, attention has been focused on thermal transfer printers, particularly dye thermal transfer printers capable of printing clear full-color images. Dye thermal transfer printers form images by superimposing a dye layer containing dyes of an ink ribbon onto an image receiving layer (hereinafter referred to simply as a “receiving layer”) containing a dye-dyeable resin on a receiving sheet, and then by transferring the desired density of the dye on the desired spots of the ink ribbon dye layer to the receiving layer with the heat supplied from a thermal head etc. The ink ribbon comprises a three-color dye layer consisting of yellow, magenta and cyan or a four-color dye layer comprising black in addition to the above. Full-color images can be obtained by transferring repeatedly and sequentially the dye for each color of the ink ribbon onto the receiving sheet. The dye thermal transfer method is replacing the silver salt photograph as a method that enables the recording of high quality images, and also as a method that enables digital printing with the recent prevalence of digital cameras.
As a method for improving the density, image quality etc. of recorded images, there has been proposed a method of using an intermediate layer that employs foaming particles on the substrate (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 1-27996 (page 1) and Japanese Unexamined Patent Publication (Kokai) No. 63-87286 (page 1)). Insulating, smoothing and cushioning properties are essential characteristics in order to use heat from the thermal head for image printing, and significantly affect the quality of the images printed and the density of the images. Specifically, when printing images, a receiving sheet is brought into contact with the thermal head via the ink ribbon, and is pressed by a rubber roll called the platen roll from the other side. A receiving sheet having a good cushioning property comes into complete contact with the ink ribbon due to the pressure applied by the rubber roll, and thereby attains the even transfer of the ink leading to a good image quality, whereas voids are formed in a receiving sheet having a poor cushioning property and ink transfer becomes disturbed in the voids, producing unevenness in the images. Thus, the cushioning property is one of the most important qualities for the receiving sheet.
Furthermore, in order to enhance image quality, an intermediate layer that utilizes hollow particles of a specific size has been proposed (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 9-99651 (pages 2-4)). However, in the receiving sheets formed by these methods, the dye that was transferred to the receiving layer penetrates into the lower layer and then diffuses up to the intermediate layer (hereinafter referred to as “blurring”), producing blurred images, and thus the image retaining property is unsatisfactory. Thus, in order to prevent the blurring of receiving sheets having an intermediate layer containing foaming particles, a protective layer (also called a barrier layer) having a high barrier property is essential.
In order to prevent the penetration of coating components of the receiving layer or solvents in the coating of the receiving layer, there has been proposed a method of introducing a layer containing a plate-form inorganic dye having an aspect ratio of 5-90 on an primer coating layer (intermediate layer) containing hollow particles (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 6-227159 (page 2)). However, it is not sufficient to prevent the penetration of image-forming dyes into the intermediate layer and, in the case of dyes for sublimation thermal transfer, there is practically no effect of preventing blurring since it is penetration on a single molecule level. Thus, in order to prevent blurring, the coated amount of the barrier layer should be increased, but excessive increases in the coating amount of the barrier layer may reduce the flexibility of the barrier layer and may cause cracking when the receiving sheet is bent to thereby reduce the commercial value. Also, the insulating effect of the intermediate layer may decrease, and the printing density may decrease leading to blurred images. In the on-going replacement of the silver salt photograph in recent years, receiving sheets having a high image quality and a high image retaining property are being sought, and there is a demand for a better technology.
Similarly, as a method of preventing the penetration of coating components of the receiving layer and solvents in the coating of the receiving layer, a barrier layer that combines polyvinyl alcohol with a polyurethane resin has been proposed (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 11-34515 (pages 2-4)). However, the above diffusion of the dye into the intermediate layer has not been taken into consideration, and the image retaining property is not sufficient. Besides, with the above combination of polyvinyl alcohol and a polyurethane resin, there is no flexibility of the barrier layer and cracking may easily occur thereby reducing the commercial value.
It is preferable that the intermediate layer (corresponding to the barrier layer) is formed from a coating solution comprising an aqueous solution of a water-soluble resin, and a dispersion of a resin or an emulsion of a resin, wherein the resin has a glass transition temperature of −30° C. to 20° C. (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 8-25813 (page 2)). Though such a barrier layer has an excellent plasticity, however, diffusion of the dye into the intermediate layer has not been taken into consideration, and the image retaining property was not sufficient and the barrier property against solvents was not perfect.
Furthermore, a barrier layer that uses an ethylene vinyl alcohol copolymer alone has been proposed (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 7-89244 (page 2)). With the use of an ethylene vinyl alcohol copolymer alone, the barrier property at a high temperature is not sufficient, and cannot fully prevent the diffusion of the dye from the receiving layer to the substrate.
Also, as described above, an improvement in transfer density resulting from the insulating effect has been proposed. It introduces an intermediate layer containing hollow particles in between the support and the receiving layer using papers as the support substrate of the receiving sheet (see the above Japanese Unexamined Patent Publication (Kokai) No. 1-27996 and Japanese Unexamined Patent Publication (Kokai) No. 63-87286). However, papers vary in stiffness with humidity and, under the condition of low humidity, stiffness may be increased with a result that the uneven thickness of papers can cause uneven density during image printing, and thereby the mere introduction of the intermediate layer cannot fully improve transfer density.
Also, there has been proposed a thermal transfer receiving sheet that has been imparted with a cushioning property and insulating property and that has no white spots or uneven density during printing by using a sulfite pulp as the above substrate for the support (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 8-2123 (page 2)). However, the sulfite pulp has a disadvantage that it has a low strength, its effect of improving unevenness in paper thickness is insufficient and, specifically, it cannot overcome the unevenness in image printing resulting from uneven thickness under the condition of a low humidity.